[v8volvo]

Late 940/960 came with the Eaton G80 locker mentioned above so that is the bolt-in option, but not everyone likes them for a couple of reasons, they only operate up to 25mph or so without modification, and their lockup action is fairly abrupt so not ideal for maintaining traction in snow. The other consideration with getting one out of a later car is that the later axles use a different tone ring with more teeth for the speedo pickup, so speedometer drive will not work on an earlier car. Probably wouldn't be a major challenge to retrofit other late style parts to an early car, might just need to change the speedometer head and use the late style pickup, but it would be more involved than just swapping the rearend and gains you the G80 which is arguably not much of a prize depending on what you want it to do.

Rear end ratio choices may be limited as well, though from what I recall the early 960 wagons all had the lockers and also had the tall 3.31 gear set, which I think would be an improvement over the stock 3.54 gearing with the M46. From your original post it sounds like the locker might have been available as an option in earlier cars too but I don't think many of them had it, probably would take a long time to find. They are common in the later ones though.

The truetrac linked above seems to be the favorite alternative for good performance. Again the challenge is getting the speedo drive working but there are solutions out there, either cutting off the stock ring and tacking it onto the truetrac, or a few folks sell precut strips that can be wrapped around the unit. Someone on turbobricks had some of these available recently. The pickup may need to be spaced outward to clear the chopper wheel too. It can all be done though.

I want to get one of these in this year too, since even with good tires the cars are helpless putting down power on uneven ground. Keep us posted on what you find out.